Electricity storage devices such as lithium-ion secondary batteries have been undergoing active development in recent years. An electricity storage device is usually designed with a microporous film (separator) between positive and negative electrodes. The separator has the function of preventing direct contact between the positive and negative electrodes, while also allowing ions to pass through the electrolyte solution that is held in the micropores.
A separator having a layer comprising an inorganic filler and a resin binder (hereunder also referred to as “porous layer” or “filler porous layer”) disposed on the surface of a separator base material has been proposed, with the purpose of imparting various properties to the separator while ensuring electrical characteristics and safety when used in a lithium-ion secondary battery (PTL 1).
PTL 1 describes coating the separator with a resin composition containing an inorganic filler and polymer particles formed from a first monomer with an acidic functional group, a second monomer with an amide group and a third monomer with a polyoxyalkylene group, to form a protective layer on the separator (see Synthesis Example 6, Formulation Example 6 and Example 16).
Polyalkyleneglycol groups have been known in the prior art as functional groups that increase solid electrolyte ion permeability, and such groups are commonly used in an organic solvent if they are to be incorporated into a copolymer for a non-aqueous electrolyte solution. Emulsion polymerization is the common method employed to obtain aqueous copolymers. When polyalkyleneglycol groups are incorporated into an aqueous copolymer, because polyalkyleneglycol groups are hydrophilic, if used in large amounts they can cause problems such as preventing formation of a particulate copolymer aqueous dispersion or interfering with removal of water in the drying step, during emulsion polymerization. In addition, since the separator base material, which is typically a polyolefin microporous film, has the property of contracting when heated, it is difficult to carry out heat drying at 100° C. or higher during formation of the electrode active material layer, and removal of the water must be carried out at low temperature.